You've done a good job building ranking but frankly the site is primarly just a hook into their data and they can do as they please with it. In essence, you're value is selling the SEO and traffic, which they would have arguably have built themselves anyways. They do likely have a solid ICANN action against you to take the domain since it incorporates their trademark and the use is demonstrably a complementary function to their site.

I'd determine the actual _cost_ of time you put into the site and ask them, nicely, to purchase it from you based on that (and the goodwill you've generated for them by making the site available). Estimate, fairly, the cost you've incurred and a fair amount for your hourly time efforts and counter offer them that.